RDF Topics Finish up XML. What is RDF? Why is it interesting? - - PowerPoint PPT Presentation

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RDF

• Topics
• Finish up XML.
• What is RDF?
• Why is it interesting?
• SPARQL: The query language for querying RDF.
• Learning Objectives:
• Identify data management problems for which RDF is a

desirable representation.

• Design RDF stores for a given data management problem.
• Write queries in SPARQL

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XQuery Evaluation Model

• The FOR clause acts more or less like a FROM clause in SQL
• You can have multiple variables and paths:

For $v1 IN path1 $v2 IN path2

• The query iterates over all combinations of values from the

component XPath expressions.

• Variables $v1 and $v2 are assigned one value at a time.
• The WHERE and RETURN clauses are applied to each

combination.

• The LET clause is applied to each combination of values

produced by the FOR clause.

• It assigns to a variable the complete set of values produced by its

path expression.

• The RETURN clause is similar to a select clause.
• Identifies the pieces that you actually want to return.

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Berkeley DB XML

• What is it?
• A native XML database built atop Berkeley DB.
• Think of BDB as the storage layer and XML/XQuery as the

schema and query layers.

• Concepts
• Document: A well-formed XML document, treated like a

row/tuple in an RDBMS.

• Container: a collection of related documents (kind of like a

table).

• Can combine different document types in a container (unlike

a relational table).

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Containers

• Include documents and meta-data
• Indices
• Dictionaries mapping element and attribute names to ID

numbers.

• Statistics to aid in optimization
• Document meta-data
• Two flavors of containers:
• Whole document: each key/data pair contains an entire

document.

• Node containers: key/data pairs correspond to elements.

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Indices

• Users can specify indices on containers.
• Three types of indices:
• Presence: keeps track of locations of elements or attributes

with a specified name.

• Useful for queries that access elements or attributes of a particular

name, regardless of value.

• Equality: indexes the values of all the elements/attributes

with a specified name

• Useful for queries on value equality or range
• Substring: indexes on substrings of the values of all

elements or attributes with a specified name

• Useful for queries using the Xquery contains() function (comparable

to LIKE in SQL).

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Using DBXML to Query

• DBXML is case sensitive and wants keywords in lower case

(select, not SELECT).

• Getting Started:
• Copy imdb.dbxml into your current directory.
• Start dbxml:

% dbxml

• Open container

dbxml> openContainer imdb.dbxml

• Specify the collection to use using:
• collection(“imdb.dbxml”)/
• Must use query command to issue an XQuery:

dbxml> query ‘for $m in collection(“imdb.dbxml”)/movie return $m/title’

• Use print to display results:

dbxml> print

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What is RDF?

• Resource Description Framework
• Grew out of the Semantic Web; way of attaching meta-data to

the web.

• Fundamental idea is to attach labels to the edges between

nodes.

• Structure is represented as a collection of triples: node, edge,

node.

• Nodes represented by URIs (uniform resource identifiers)
• Can be used to represent both structured and semi-structured

data.

• Perhaps more importantly, interestingly, it is a natural

representation for graphs.

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RDF Concepts

• Things are called resources.
• Resources have properties.
• Properties have values.
• You can describe a resource by making statements about it;

these statements define its properties.

• In RDF, we use the terms subject, predicate, and object to

describe the pieces of these statements.

• The statement, “Margo has brown hair.” has:
• Subject=resource: Margo
• Predicate=property: hair
• Object=value: brown

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Naming

• RDF uses URI references (URIref) to identify any of

subject, predicate, object.

• A URIref is a URI with an optional fragment identifier

(signified by the # symbol).

• Examples:
• URI : http://www.eecs.harvard.edu/~margo/somepage
• URIref: http://www.eecs.harvard.edu/~margo/somepage#foo
• Although RDF was invented for the web and is

specified in terms of URI’s, you can use it to store any data and refer to subjects, predicates, and

• bjects by any ID of your choosing.

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The Model

• Triples
• That’s right -- the entire data model is described by triples of

subject, predicate, object, and by that mechanism, you can build up arbitrarily complex objects and relationships.

• Think of RDF as describing a graph where the

subject and object are nodes and the predicates are edges.

• So, our,”Margo has brown hair,” statement would be

represented as (margo, hair, brown) and look like: Margo “brown” hair

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Drawing Pictures of Data

• Although there are XML representations of RDF, it is

by far most natural to think of RDF visually as a graph.

• When we think about the graph stucture, we make

URIs circular nodes and literal values rectangles.

• So our previous example might look like:

MyURI “Margo” “brown” hair name

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Let’s Draw our (tired) Products

Product itemname Vendor name sku-value # $$$

item inventory vendor sku instock val curr

currency

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Making this more RDF-like

SKU itemname # $$$

name inventory vendor instock val curr

currency vendor

price

name address

name address products

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Queries over RDF: SPARQL

• Yet another query language.
• Like many others, looks a lot like SQL:
• Basic Structure:

SELECT <what to return> FROM <what collection> WHERE <what patterns to match>

• Variables
• Name is preceded by a ? (or $- in Jena)
• Predicates are expressed as pattern-matching on

triples.

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WHERE Claues

• WHERE { triple . triple . … };
• Triple is a combination of variables, URIs, constants
• The “.” means AND
• Example: find all products sold by the vendor whose

ID is: vend001

SELECT ?name WHERE {

<vend001> <products> ?p> . <?p> <name> <?name>

};

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UNION

• If the “.” operator is AND, how do you specify OR?
• Find all products in either US or Canadian dollars:

SELECT ?name WHERE {

?sku <inventory> ?inv . ?inv <price> ?price . {{ ?price <curr> “USD” } UNION { ?price <curr> “CAD” }} . ?sku <name> ?name

};

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Optional Matches

• Let’s say that we want all items and their vendor, but if there is

no vendor specified, we want to see the item anyway.

• The obvious query doesn’t quite work:

select ?name ?vendor where { ?product <vendor> ?vendor . ?product <name> ?name };

• Use OPTIONAL to return items without a vendor:

select ?name ?vendor where { ?product <name> ?name . OPTIONAL { ?product <vendor> ?vendor } };

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Filters

• Let’s you impose constraints on values that match a

pattern.

• Syntax: part of the WHERE clause (add like you

would add a pattern):

• FILTER expression
• Example: Find all product over $1.

select ?name ?val where { ?product <name> ?name . ?product <inventory> ?inv . ?inv <price> ?price . ?price <val> ?val . FILTER (?val > “1.0”) };

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Functions on which you can filter

• There are many functions on which you can filter:
• Regular binary and unary operators:
• ! (not), || (or), && (and)
• Comparators (numeric and string): <, > , =, !=, <=, >=
• REGEX: regular expressions
• BOUND: tells you if a variable is bound to a value.
• Regular Expressions
• regex(string, pattern)